JPH07233449A - Ferritic stainless steel sheet and its manufacturing method - Google Patents

Abstract

(57) [Abstract] [Purpose] A low Ti content that combines workability and corrosion resistance.
Provide Cr ferritic stainless steel sheet. [Composition] C: 0.015% or less, N: 0.015% or less by weight%, C + N: 0.020% or less, Cr: 10 to 14%, S
i: 2.0% or less, Mn: 2.0% or less, P: 0.04% or less, Al:
0.1% or less, S: C x 0.5 to 0.010%, Ti: 0.02 to 0.2
0%, but Ti / (C + N): 2.0 or more A low Cr ferritic stainless steel sheet having excellent workability and corrosion resistance, having a steel composition consisting of the balance of Fe and inevitable impurities.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a ferritic stainless steel sheet excellent in workability and corrosion resistance and a method for producing the same, and more specifically, a low Cr ferritic stainless cold-rolled steel sheet excellent in workability and corrosion resistance and heat treatment. The present invention relates to a rolled steel sheet and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, stabilizing elements Ti and Nb with low C and N
It is known that a perfect ferritic stainless steel sheet containing, for example, etc. exhibits excellent workability. Above all, the amount of Cr is 10
The low Cr ferritic stainless steel sheet with Ti as the stabilizing element at -14% is soft and has excellent elongation and deep drawability, so workability is required and it is used in a relatively mild wet environment. It is widely used as a material for parts and as a material for members used at temperatures of 800 ° C or lower.

By the way, as the ferritic stainless steel sheet, a cold rolled steel sheet is usually used, and unlike carbon steel, hot rolled products are not frequently used. One of the reasons for this is
This is because the ferritic stainless steel is difficult to recrystallize during hot rolling, so that the cast structure is not sufficiently broken and exhibits nonuniform material properties, and the coarse structure tends to reduce toughness.

By the way, a normal ferritic stainless steel sheet is subjected to hot rolling, hot rolled sheet annealing, cold rolling once, or cold rolling twice or more with intermediate annealing interposed, and final annealing. Becomes However, today, various attempts have been made even in the process of not performing annealing in order to reduce the manufacturing cost. A problem with such a method in which the annealing is omitted is a method of improving the workability of the cold-rolled steel sheet, and for that purpose, until now, mainly the working conditions of the hot rolling have been devised.

Therefore, as a technical problem in the ferritic stainless steel sheet, under the circumstances where the reduction of the manufacturing cost is strongly demanded today, the production of the ferritic stainless cold rolled steel sheet in which the annealing of the hot rolled sheet can be omitted. Establishing the technology, and if possible, establishing the manufacturing technology of the ferritic stainless hot rolled steel sheet that can be used as it is as a hot rolled sheet.

[0006] Here, the following is an overview of the prior art regarding the method for improving the workability of ferritic stainless steel. First, if the cast structure is not sufficiently broken, it may cause surface undulation after working of the cold rolled steel sheet, which is called ridging or roping. Therefore, in order to solve these problems, it is important to promote recrystallization during hot rolling.

As a method of accelerating this recrystallization, strong working in the vicinity of 1000 to 1100 ° C. is performed on a steel sheet having a two-phase structure of ferrite-austenite at high temperature and a steel sheet having a ferrite single phase but containing no stabilizing element. It is known to do so (for example, "Recrystallization Behavior of 17Cr Ferritic Stainless Steel During Hot Rolling" Iron and Steel, 1969 (1983) p.1440).

On the other hand, in order to improve the intergranular corrosion resistance of a ferritic stainless steel sheet containing a stabilizing element, generally,
Ti or Nb is contained 10 to 20 times or more the weight of C + N, C and N are fixed as precipitates, and solid solution C and N are reduced. So much Ti or
Addition of Nb generally enhances deep drawability, but on the other hand, makes recrystallization less likely to occur during hot rolling. For this reason, the problems of ridging and roping are likely to occur in cold-rolled steel sheets, and the same type of problems are likely to occur in hot-rolled steel sheets.

As described above, conventionally, among ferritic stainless steel sheets containing a stabilizing element, a steel sheet having excellent workability (recrystallizing property) mainly having ridging and roping properties and excellent intergranular corrosion resistance is known. Has not been done.

Further, for example, C: 0.05% or more, Cr: 16%
Several technologies have been disclosed for steel types having the composition of No. 1 and having a two-phase mixed structure of ferrite-austenite during hot rolling. For example, JP-A-52-95527 discloses a method of lowering the yield strength and increasing the elongation by winding at a high temperature of 850 ° C. or higher. Further, Japanese Patent Laid-Open No. 57-70231 discloses a method of adding Al and winding at a low temperature of 700 ° C. or less.

However, in the case of a ferrite single-phase Ti-containing low-Cr ferritic stainless steel sheet, if an attempt is made to manufacture a cold-rolled steel sheet by omitting the hot-rolled sheet annealing, even if the cold-rolled steel sheet is produced during hot rolling as described above, There is a problem that elongation and deep drawability are deteriorated even when high-temperature coiling or low-temperature coiling is performed as in the case of a steel type having a two-phase mixed structure of austenite.

Although it is known that the elongation may be improved to some extent by lowering the Ti and Nb contents,
This time, the problem is that the solute C and N in the portion heated to high temperature such as near the welded part increases and the intergranular corrosion resistance deteriorates. Eventually, hot work sheet annealing is omitted and high workability is achieved. It was difficult to obtain a Ti-containing low Cr ferritic stainless cold-rolled steel sheet that also had corrosion resistance. Further, it was even more difficult to obtain a Ti-containing low Cr ferritic stainless hot-rolled steel sheet having both high workability and corrosion resistance by omitting cold rolling.

[0013]

As described above, conventionally, there has been no known steel sheet containing a Ti-containing low Cr ferritic stainless steel sheet having a component system having both workability and corrosion resistance. Further, especially when the hot rolled steel sheet is not annealed in the manufacturing process of the steel sheet, the elongation of the steel sheet is reduced, and further, when cold rolling is not performed, surface undulation such as ridging and roping occurs. There was an occasion.

An object of the present invention is to provide a low Cr ferritic stainless cold-rolled and hot-rolled steel sheet which has Ti and has both workability and corrosion resistance. A further object of the present invention is to provide a method for producing a low Cr ferritic stainless steel cold-rolled steel sheet which is excellent in workability and corrosion resistance even without annealing the hot-rolled steel sheet.

In addition, it is an object of the present invention that Ti which is excellent in workability and corrosion resistance can omit the cold rolling step.
An object of the present invention is to provide an added low Cr ferritic stainless hot rolled steel sheet and a manufacturing method thereof.

[0016]

[Means for Solving the Problems] In order to achieve the above-mentioned objects, the present inventors have studied a method of enhancing intergranular corrosion resistance, pitting corrosion resistance, elongation, and deep drawability of low Cr ferritic stainless steel sheets. As a result, it was newly found that the purpose can be achieved by adding a certain amount of S under the condition of a smaller amount of Ti, C and N. In addition, with this new steel sheet, even when hot-rolled sheet annealing is omitted to produce cold-rolled steel sheet, further cold rolling is omitted and hot-rolling is performed. It turned out that the sex can be obtained.

The gist of the present invention is, by weight, C: 0.015% or less, N: 0.015% or less, provided that C + N: 0.020% or less, Cr: 10 to 14%, Si: 2.
0% or less, Mn: 2.0% or less, P: 0.04% or less, A
l: 0.1% or less, S: C × 0.5 to 0.010%, Ti: 0.02 to
0.20%, but Ti / (C + N): 2.0 or more A ferritic stainless cold-rolled steel sheet excellent in workability and corrosion resistance, having a steel composition in which the balance is Fe and inevitable impurities.

From another point of view, the gist of the present invention is, by weight%, C: 0.015% or less, N: 0.015%.
Below, but C + N: 0.020% or less, Cr: 10-14%,
Si: 2.0% or less, Mn: 2.0% or less, P: 0.04%
Below, Al: 0.1% or less, S: C × 0.5 to 0.010%, T
i: 0.02 to 0.20%, but Ti / (C + N): 2.0 or more A ferritic stainless hot rolled steel sheet having excellent workability and corrosion resistance, having a steel composition in which the balance is Fe and inevitable impurities.

According to a preferred embodiment of the present invention, in any of the above cold-rolled steel sheet or hot-rolled steel sheet, Cu: 0.
5% or less, V: 0.5% or less, Ni: 0.5% or less, and M
It may contain one kind or two kinds or more selected from the group consisting of o: 1.0% or less.

According to another aspect of the present invention, a ferritic stainless steel having the above steel composition as a cold rolled steel sheet is hot-rolled, wound at 600 ° C. or higher, and then hot-rolled sheet annealing is not performed. A method for producing a ferritic stainless cold-rolled steel sheet excellent in workability and corrosion resistance, which is characterized by performing cold rolling.

Further, the present invention is characterized in that a ferritic stainless steel having the above-mentioned steel composition as a hot rolled steel sheet is hot-rolled and wound at 600 ° C. or higher, which is excellent in workability and corrosion resistance. It is a manufacturing method of a stainless hot rolled steel sheet.

In a further preferred embodiment of the present invention, when producing a hot-rolled steel sheet, in the case of containing at least one of the above Cu, V, Ni and Mo, the coiling temperature is 650.
It may be higher than or equal to ° C.

[0023]

In the present invention, the reason why the steel sheet is defined as described above will be described in detail below together with its action. In the present specification, unless otherwise specified, “%”
Is "% by weight".

C, N: C and N are each 0.015% or less and C + N≤0.02% to improve workability and corrosion resistance.
Below. Preferably, C: 0.010% or less, N: 0.010%
Below, C + N: 0.010% or less.

Cr: Cr is a main element responsible for the corrosion resistance of stainless steel, and it is necessary to contain 10% or more. On the other hand, it is preferable that the amount is small from the viewpoint of workability improvement.
Limited to 14% or less. It is preferably 11.0 to 13.0%.

Si, Mn: Si and Mn are basic elements involved in deoxidation of steel and need to be added as deoxidizers, but excessive addition may impair workability, so each is limited to 2.0% or less. did. It is preferably 1.5% or less and 1.0% or less, respectively.

P: P is an element harmful to corrosion resistance, and
Limit to less than 04%. S: S is generally considered to be an element that deteriorates corrosion resistance, and is usually limited to 0.002% or less. However, in the present invention, the intergranular corrosion resistance and workability are rather reduced with the decrease of (C + N). Added to enhance.

The role of the S element added to steel in the present invention for improving the corrosion resistance and workability has not been fully clarified, but it is presumed as follows. That is, the S element forms a stable Ti carbosulfide [Ti (C, S)] together with C and Ti and reduces the solid solution C. Therefore, sensitization at the weld is prevented. In addition, the grain size of precipitates increases and the precipitation density decreases, resulting in a steel quality that is easily recrystallized, and the workability such as deep drawability and elongation is improved even with hot rolling.

As shown in the examples, the content of C is 0.
Addition of more than 5 times showed the effect of improving workability and corrosion resistance.
However, if the total content exceeds 0.010%, the single sulfide TiS precipitates and the pitting corrosion resistance is impaired. Therefore, the content of C is 0.5 times or more, and the total content is 0.0
Limited to 10% or less.

However, economically, C: 0.001% is considered to be the lower limit, so S is 0.0005% or more, but generally C ≧ 0.002%, so S ≧ 0.001%, preferably 0.00
3% or more.

The reason why the addition of S under the conditions of constant Ti, C and N contents is effective in improving the intergranular corrosion resistance and workability of the low Cr ferritic stainless steel has not been confirmed. However, it is presumed that this is due to a decrease in solid solution C due to stable formation of Ti carbosulfide and a decrease in precipitation density with an increase in precipitation particle size.

Ti: Ti is added to improve workability and corrosion resistance. In the present invention, the effect of fixing C and N is exhibited by the addition of S with a small amount of Ti. However, even if S is added in the above range, the amount of Ti is less than 0.02%, or Ti / (C
When + N) is less than 2.0, C and N in the steel are replaced by Ti (C,
The effect of fixing as S), TiC, and TiN is not sufficient, and the workability improving effect is not remarkable. On the other hand, if the content exceeds 0.20%, the steel is hardened, and the high workability targeted by the present invention cannot be obtained. Therefore, the Ti content is limited to 0.02% or more and 0.20% or less, and the Ti / (C + N) is limited to 2.0 or more, preferably 20 or less. Further, by setting the lower limit of the Ti content to 0.02% or more, it is possible to obtain a steel sheet having sufficient workability even if the hot rolling annealing and the cold rolling are omitted.

Al: Al is an element having a very large deoxidizing ability of steel, and Si and Si are also used to improve the yield of addition of deoxidizing and Ti.
It is added together with Mn. It also has the effect of increasing oxidation resistance. On the other hand, excessive addition causes hardening of the steel and may deteriorate workability, so the content is limited to 0.1% or less.

Cu, V, Ni, Mo: At least one of these elements is added, if necessary, in order to additionally improve the corrosion resistance of the ferritic stainless steel. Cu,
0.5% for V and Ni, 1.0% for Mo
If the content of Ni exceeds 0.5%, the degree of improvement in corrosion resistance decreases depending on the addition amount, and the workability decreases and the cost increases. Therefore, Cu, V, and Ni are each 0.5% or less, and Mo is 1.
It was set to 0% or less. Next, the reasons for defining the manufacturing conditions will be described.

The ferritic stainless cold-rolled steel according to the present invention may be manufactured by hot rolling, hot-rolled sheet annealing, or cold rolling by a conventional method, but more advantageously, cold-rolled sheet annealing is omitted. It can be manufactured by hot rolling or can be used as hot rolled.

The coiling temperature is 600 ° C. or higher, preferably 65
0 ° C or higher: In a preferred embodiment of the present invention, under the condition that cold rolling is performed and hot-rolled sheet annealing is omitted, as shown in Examples, 600 ° C or higher, preferably 650 ° C or higher , Because a high elongation and r value can be obtained, it was defined as above. The upper limit is not particularly specified, but it is preferably 750.
It is below ℃.

There are no particular restrictions on the hot rolling conditions and the conditions of the cold rolling performed after the above-mentioned winding, and any conventional conditions may be used. By setting the lower limit of the Ti content to 0.02% or more as described above, cold rolling can be omitted in addition to hot-rolled sheet annealing.

The microstructure of the as-hot-rolled steel sheet was observed, and the relationship between the winding temperature and the recrystallization rate is shown in FIG. In the steel I according to the present invention (see Table 3 for the composition) shown by ◯ in the figure, the recrystallization rate is high in the as-wound state. On the other hand, the comparative steel M indicated by Δ in the figure (see Table 3 for the composition) has a low recrystallization rate in the as-wound state. in this way,
The steel of the present invention has a coiling temperature of 600 ° C or higher, preferably 65 ° C or higher.
By setting the temperature to 0 ° C. or higher, a hot-rolled steel sheet with a high recrystallization rate can be obtained.

The coiling temperature is more preferably 650 ° C. or higher: In a further preferred embodiment of the present invention, in the case of a steel containing at least one of Cu, V, Ni and Mo for improving corrosion resistance,
Under the condition that hot-rolled sheet annealing and cold rolling are omitted, in addition to setting the lower limit of the Ti content to 0.02% or more, as shown in the example, high elongation and r-value can be obtained by winding at 650 ° C or more. Therefore, it is defined as follows. Next, the operation of the present invention will be described in more detail with reference to Examples.

[0040]

【Example】

(Example 1) This example shows a manufacturing method in which hot rolling and cold rolling are performed. In this example, Ti having the steel composition shown in Table 1,
Steels with different C, N and S contents are hot-rolled under the following specific conditions, with or without hot-rolled sheet annealing 1.2 mm
Cold rolled to thickness, then finish annealed (830 ~ 930 ℃ x 1
After 5 minutes), the workability and corrosion resistance were investigated. Hot rolling heating temperature: 1150 ℃ Hot rolling with 7 passes from 40mm thickness to 4.5mm thickness. Hot rolling finish temperature: 860 ℃ Slow cooling at -50 ℃ / h after coiling temperature. Hot rolled sheet annealing: 880 ° C x 3 minutes, Finish annealing: 880 ° C x
2 minutes.

The workability was investigated by using JIS 13B No. B shape tensile test pieces. Elongation uses a value in the direction of 90 degrees with respect to the rolling direction, 0, 45 for r value which is an index of deep drawability,
Mean values of 90 degree direction value _{[(r 0 + 2r 45 +} r 90) / 4] were evaluated using. Regarding the corrosion resistance, intergranular corrosion and pitting corrosion tests were conducted.

That is, the steel sheet after finish annealing was heated to 1100 ° C. and air-cooled as a treatment corresponding to sensitization, and then a test piece was sampled. The test piece was wet-polished # 320 to have a thickness of 1.0 mm, and the test piece was subjected to the test.

Regarding intergranular corrosion, the temperature of the Strauss test specified in JIS-G-0575 was changed to 60 ° C., and the test was conducted.
Grain boundary corrosion resistance was evaluated by corrosion weight loss and bending crack observation.

On the other hand, the pitting corrosion test was carried out at 50 ° C. by repeating immersion in 5% NaCl solution and pulling up (cycle of 30 minutes to 30 minutes) for 50 hours, and the maximum pitting depth was measured by an optical microscope after descaling. The pitting corrosion resistance was evaluated.

Table 2 summarizes the winding temperature of hot-rolled steel sheet, hot-rolled sheet annealing, finish annealing conditions, elongation of cold-rolled steel sheet, r value, and intergranular corrosion resistance and pitting corrosion resistance test results. Indicate. As is clear from Table 2, according to the present invention, it is possible to obtain a low Cr ferritic stainless cold-rolled steel sheet excellent in workability and corrosion resistance.

Next, in the same manner, the intergranular corrosion test of the TIG tandem welded material was conducted on the cold rolled steel sheet of 1.2 mm thickness when the steel composition A was used as the basic composition and the C content and the S content were changed. The results are shown in Figure 2. In the figure, ○: no crack, △: fine crack, ×: crack. No cracks were found for the material within the range of the invention in the shaded area.

[0047]

[Table 1]

[0048]

[Table 2]

(Example 2) This example shows an example in which only hot rolling is performed and cold rolling is not performed. Steels with different Ti, C, N, and S contents shown in Table 3 are vacuum-melted and hot-forged into a hot rolled material with a thickness of 40 mm, heated to 1100 ° C, and heated to 3.6 mm with 7 passes. Rolled. After the hot rolling, the steel sheet was spray-cooled and then inserted into a slow cooling furnace at a temperature of 750 to 500 ° C., and cooled to room temperature at a rate of −50 ° C./h corresponding to winding of a hot rolled coil. Further, after finish annealing (880 ° C. × 5 minutes), workability and corrosion resistance were investigated. Some of the steel sheets were left unwound.

The workability was investigated by the same method as in Example 1 (however, the sample was taken from the center of the steel plate and the plate thickness was 2 mm). Regarding the corrosion resistance, intergranular corrosion and pitting corrosion tests were conducted. The steel plate is ground to a thickness of 2 mm, and MAG welding is performed using a SUS309 welding wire with a wire diameter of 2 mm with the heat input to generate a back wave.
A 50 mm long test piece was taken. The test piece was subjected to the test as it was welded.

The intergranular corrosion was investigated by the same method as in Example 1. The pitting corrosion test is performed by the same method as in Example 1,
The maximum depth of pitting that appeared in the HAZ area after descaling was measured with an optical microscope to evaluate pitting resistance.

Winding temperature of hot rolled steel sheet, presence / absence of annealing of hot rolled steel sheet,
Table 4 shows the elongation, r-value, intergranular corrosion resistance and pitting corrosion resistance of hot rolled steel sheet.
Are shown together. As is clear from Table 4, according to the method of the present invention, a low Cr ferritic stainless steel hot rolled steel sheet excellent in workability and corrosion resistance can be obtained.

The relationship between the coiling temperature and the recrystallization rate for the steels I and M of this example is shown in the graph. The "recrystallization rate" was obtained by the point calculation method within the visual field of a 100x optical microscope after corroding a micro sample of the cross section in the direction perpendicular to the rolling with virella liquid.

[0054]

[Table 3]

[0055]

[Table 4]

[0056]

Industrial Applicability According to the present invention, it is possible to obtain a low Cr ferritic stainless steel sheet having both corrosion resistance and workability, and the industrial advantage is extremely high. Further, in the preferred embodiment of the present invention, even if hot-rolled sheet annealing and cold rolling are omitted,
It is possible to obtain a low Cr ferritic stainless steel sheet having both corrosion resistance and workability.

[Brief description of drawings]

FIG. 1 is a graph showing hot rolling temperature and recrystallization rate of steel sheet microstructure.

FIG. 2 S of ferritic stainless steel according to the present invention
5 is a graph showing the relationship between the C content range and the results of the intergranular corrosion test of Example 1.

Claims (7)

[Claims] 1. By weight%, C: 0.015% or less, N: 0.015% or less, provided that C +
N: 0.020% or less, Cr: 10 to 14%, Si: 2.0% or less, Mn: 2.0% or less, P: 0.04% or less, Al: 0.1% or less, S: C × 0.5 to 0.010%, Ti: 0.02 to 0.20%, but Ti / (C + N): 2.0 or more A ferritic stainless cold-rolled steel sheet with excellent workability and corrosion resistance, having a steel composition with the balance being Fe and inevitable impurities. 2. The composition according to claim 1, further comprising at least one selected from the group consisting of Cu: 0.5% or less, V: 0.5% or less, Ni: 0.5% or less, and Mo: 1.0% or less. The described ferritic stainless steel cold rolled steel sheet. 3. A ferritic stainless steel having the steel composition according to claim 1 or 2 is hot-rolled, wound at 600 ° C. or higher, and then cold-rolled without hot-rolled sheet annealing. And a method for producing a ferritic stainless cold-rolled steel sheet having excellent workability and corrosion resistance. 4. By weight%, C: 0.015% or less, N: 0.015% or less, provided that C +
N: 0.020% or less, Cr: 10 to 14%, Si: 2.0% or less, Mn: 2.0% or less, P: 0.04% or less, Al: 0.1% or less, S: C × 0.5 to 0.010%, Ti: 0.02 to 0.20%, Ti / (C + N): 2.0 or more Ferrite-based stainless hot rolled steel sheet with excellent workability and corrosion resistance, having a steel composition with the balance being Fe and inevitable impurities. 5. The composition according to claim 4, further comprising one or more selected from the group consisting of Cu: 0.5% or less, V: 0.5% or less, Ni: 0.5% or less, and Mo: 1.0% or less. The described ferritic stainless hot rolled steel sheet. 6. A ferritic stainless steel having the steel composition according to claim 4, which is hot-rolled and wound at 600 ° C. or higher, to produce a hot-rolled ferritic stainless steel sheet having excellent workability and corrosion resistance. Method. 7. A ferritic stainless steel hot rolled steel sheet having excellent workability and corrosion resistance, which is characterized in that the ferritic stainless steel having the steel composition according to claim 5 is hot rolled and wound at 650 ° C. or higher. Method.